In addition to the tznius standards found in the Gemara women are free to increase their tznius. However when a particular manner of tznius becomes widespread within a community, each woman within that community must abide by those standards.
So there is no halachic reason why Bais Yaakov girls can’t dress as you suggest. However because they do not dress that way it would be untzniusdik for an individual girl to dress in a such a fashion.